Sensors, as the term is used here, refer to systems that react to a change in the environment. Pressure sensors react to an applied force or pressure using a variety of physical principles. Optical sensors change their optical properties under applied force. Similarly, electrically resistive, or simply resistive, sensors have an electrical resistance that changes under applied force. Piezoresistive sensors measure the change in electrical resistance of a piezoresistive material as pressure is applied.
Capacitive sensors change capacitance. This can be in response to an applied force; it can also be in response to the proximity of an object with relatively large capacitance, such as a person. Capacitive sensors can also use a combination of resistive and capacitive sensing, in which the electrical resistance is measured when the capacitance changes.
Capacitive sensors are known and are used, for example, in touch screens and elevator buttons. The change in capacitance is typically based on one of two principles. The first approach involves changing the capacitance monitored by the sensing system through direct electrical contact with a large capacitive object, usually a person through their finger. In certain cases this type of sensor may also function to detect the proximity of an object to the touch sensor, not requiring physical contact with the touch sensor. Because these systems often require direct contact between the person and the sensing system they may not work if, for example the person is wearing a glove. Additionally, capacitive coupling may not be well suited to quantitatively measuring the applied pressure or proximity, but are capable of binary (on/off) sensing.
The second approach uses two conductive planes separated by a compressible, resilient dielectric. This composite forms a capacitor whose capacitance depends in part on the distance between the conductive planes. The compression of the dielectric under pressure changes the capacitance between the planes, which can be detected by the sensing system. By calibrating the compression with the applied force or pressure, this system can be used to quantify the force or pressure of the interaction with the sensor.
In recent years, there has growing interest in so-called “smart fabrics” that give electronic devices physical flexibility. They allow an electronic device to be incorporated into an existing fabric rather than have a separate electronic device. An example of a smart fabric is a computer keyboard that can be rolled up when not in use.
Flexible sensors are needed for smart fabrics and other applications that require flexibility. Flexible optical pressure sensors have been described, for example, in U.S. Pat. No. 4,703,757 to Cohen and U.S. Pat. No. 5,917,180 to Reimer & Danisch. Flexible sensors based on electrical contact of two or more conducting planes are available from Eleksen Ltd. of Iver Heath, United Kingdom. Flexible pressure sensors that use principles of piezoreistance are available from Softswitch Ltd. of Ilkely, United Kingdom. A flexible capacitive sensor based on the capacitance of the human body is described in U.S. Pat. No. 6,210,771 to Post, et al. A flexible capacitive sensor that uses the change in spacing between conductive planes is described in a series of US patents to Goldman, et al. These patents teach the use of flexible conductive and dielectric layers, but they do not teach a system which can be used to determine location, nor do they teach systems with multiple sensors (beyond the simple case of replications of a single sensor). All patent documents referenced in this specification are hereby specifically incorporated by reference in their entirety as if fully set forth herein.
Thus there remains a need for a large-area flexible capacitive pressure sensor with good spatial resolution, capable of quantifying applied pressure or force. Here we address those issues by describing multiple methods of constructing a flexible capacitive sensing system with multiple sensors that detects the presence of an applied force or pressure and is capable of determining the magnitude and location of the applied force or pressure.